1. Field of the Invention
The invention is in the field of radiation therapy and more specifically related to the use of leaf apertures to control delivery of radiation to a patient.
2. Related Art
Radiation, e.g., x-rays or particle beams, is used in a variety of medical treatments. Preferably, radiation is delivered to a treatment volume and not to a surrounding region of healthy tissue. In order to achieve these treatment preferences, a radiation beam is typically shaped using an aperture. The aperture is configured to shape the radiation beam according to a geometric projection of the treatment volume along the axis of the radiation beam.
One type of aperture includes a series of movable leaves. These leaves may be comprised of a series of thin plates whose thin dimension and axis of movement are both perpendicular to the direction of the radiation beam. Movement of the leaves allows for the generation of a variety of openings and, thus, a variety of radiation beam shapes. The resolution of these shapes is, in part, dependent on the thickness and number of the plates.
In some treatment schemes, a radiation source and aperture are rotated around the treatment volume, i.e., around a patient, using a gantry. One advantage of this approach is that the radiation beam does not always pass through the same healthy tissue. For example, at two different points in the rotation process, the radiation beam may be directed at the treatment volume from directions that differ by approximately ninety degrees. While the radiation beam passes through the treatment volume from both directions, it passes through different parts of the healthy tissue from the different directions. As such, the harmful impacts on the healthy tissue are distributed and their total magnitude reduced.
In the prior art, the rotating radiation source and aperture are configured to provide a single fluence. The fluence is a two-dimensional object defined in a plane. An intensity is defined at each point of fluence. The intensity is the amount of radiation per unit cross-sectional area. The magnitude of the fluence may take any of a range of continuous values. However, magnitude of the fluence is typically approximated by discrete fluence levels, i.e. the continuous fluence values are mapped to a set of discrete values.
The fluence is distinct from dose which is the total radiation received by a part of the treatment volume. The dose is a three-dimensional distribution whose magnitude varies as a function of the position within the treatment volume. This variation may be undesirable. In the prior art, improved control over the three-dimensional dose distribution inside the treatment volume may be achieved by rotating the aperture around the treatment volume more than one time. The intensity and spatial distribution of radiation provided by a beam source is changed between consecutive rotations. This may be accomplished, for example, by changing the shape of an aperture within the beam source. The availability of more than one fluence per position along the trajectory of the rotation allows for improved control over the dose distribution. However, this approach results in a variety of disadvantages, for example, rotation of the aperture more than once around the treatment volume requires additional time.
When the radiation source and aperture are rotated around a treatment volume, the projection of the treatment volume onto the aperture may change. As such, it is desirable to change the opening within the aperture as the aperture is moved though different angles. These changes are typically achieved by moving leaves within the aperture to positions determined prior to the treatment. There are several ways in which leaf positions may be determined. In one approach, the projection of the treatment volume along the axis of the radiation beam is determined at each position along the arc of rotation. Two points along the arc of rotation approximately 180 degrees apart will have essentially the same projections (e.g., a mirror image thereof) and, therefore, essentially the same relative leaf configurations. Radiation generated by a beam source is typically constant as the aperture is rotated around the treatment volume. The amount of radiation received by various parts of the treatment volume is, therefore, primarily controlled by the aperture.
One approach to determining leaf positions that can include consideration of the dose received by each part of the treatment volume includes selecting a particular set of leaf positions, for example, by using projections of the treatment volume. The dose distribution throughout the treatment volume is then calculated. After this calculation, the selected set of leaf positions is changed slightly and the projected dose distribution is again calculated. If the change in leaf positions results in an improvement in the dose, the changed leaf positions are kept and the process is repeated. This iterative process may be facilitated by statistical or optimization algorithms. Disadvantages with this approach include lengthy computation times.